This invention relates to an ultrasonic imaging apparatus used for medical diagnosis, material crack detection and marine investigation.
In order to raise the level of image performance represented by image signal to noise ratio and azimuth resolution in an ultrasonic imaging apparatus based on an array type probe in which an ultrasonic beam is converged to form an image by controlling phase and amplitude of transmission/reception signals of individual elements constituting the array, a great number of elements need to be allocated to a transmission/reception aperture of the probe by increasing the number of signal channels undertaking simultaneous signal processing. Disadvantageously, awful expensiveness results from an increase in the number of signal channels especially reception channels.
A countermeasure to this problem is disclosed in, for example, U.S. Pat. No. 4,235,111. In this patent, among a great number of elements within a reception aperture, a plurality of elements near the center are connected in common and signals of the plural elements are treated as a single channel signal. For elements at a peripheral portion of the reception aperture, on the other hand, such elements as symmetrically positioned are paired and connected in common so that each pair provides a single channel signal. The respective channel signals are separately delayed and controlled in phase, and the phase controlled signals are added together. The resultant signal stands for a reception signal representative of a reception beam having a focal point at a desired depth.
When advancing the expediency proposed by the above patent by increasing the number of elements to be connected in common to the same channel, however, the magnitude of an unwanted response due to a side lobe of a reception beam is increased in proportion to an increase in the number of elements commonly connected to the same channel, thus degrading the signal to noise ratio of the image.